In 1964, the Texan folksong collector James Ward Lee noted that for every "real" folk song a collector found, he'd get six or eight "penny dreadfuls", or pop songs. "One of the things highbrow collectors do not like to admit," he concluded, "is that their informants have no taste whatsoever."

One of the main tasks folk song collectors have always faced is choosing which of the many songs their informants sing are folk songs and which aren't. Most of them have thought this a relatively easy task: folk songs are uncommercial, pure products of a shared heritage, passed on from generation to generation, whereas pop songs are outside interlopers, invasive species that endanger the survival of the genetically unmodified, authentic, living tradition.

But it's not always so clear, especially when informants claim that all their songs are traditional, which, since they are usually being paid per song, they usually will. It is up to the collector to apply discriminatory skills and ferret out those songs of greatest value in defining a range of folklore untainted by outside influences.

Every performer has outside influences, even if they are only birdsong and river sounds, though more commonly they would have come from the next town over or a visiting traveller. These influences are more likely to be perceived as "outside" if they are similar to music from another tribe, region or race. The object of the folk song collector, then, has historically been to keep the tradition ethnically clean. The ethnically "dirty" songs are labelled "pop junk" and quickly expunged.

But that ambition to create a tradition of the untainted folk song is not only ethically suspect, but quixotic. Every folk song, no matter how old, was composed by a real person, and its transmission usually involved some sort of border crossing. In other words, the folk song collector would have to apply his or her own ideas about the purity of a certain heritage, to label some songs as folk and others as junk.

So what exactly does folk music purport to be? Nowadays, it's almost anything at all - Lester Bangs called Lou Reed's Metal Machine Music "folk", and if you're playing steel-string acoustic guitar, you're almost automatically a folk musician, even if you've never heard a song that predates the Beatles. But the term "folklore" was coined in 1846, and its anthropological definition is, more or less, the orally transmitted expression - often anonymous, unselfconscious and spontaneous - of a small homogenous group with a long common tradition. It's certainly not hard, then, to call the music of the Navajo or the Ba-Benzélé pygmies "folklore".

In the early 20th century, however, problems arose. The main one was that cultural integration had all but eliminated the purity of most of the groups in Europe and America producing "folklore".

But most folklorists assumed that distinct and culturally separate groups ranging from American blacks to Appalachian whites still existed, despite the evidence that their music had undergone countless transformations through the mixing of traditions. John Lomax, who, along with his son Alan was the premier collector of American folk music, embarked on his monumental quest for black American folk songs in 1933 by defining them as the "songs that are ... the least contaminated by white influence or by modern negro jazz". What Lomax was really after, though, he had revealed a year earlier: he wanted to feel "carried across to Africa ... as if I were listening to the tom-toms of savage blacks". (Remember that at the time he was writing this, "black" was a derogatory term.) In other words, when deciding which songs were "most unlike those of the white race", Lomax would always choose the most primitive forms of expression, disregarding the jaw-dropping complexity and sophistication of much of the black music of his time.

The "white influence" was, of course, impossible for Lomax to escape. In the Southern black penitentiaries, where he assumed the prisoners would "slough off the white idiom they may have employed", his informants inevitably sang garbled versions of songs of black, white, and mixed origin, distantly remembered from their days of freedom. Lomax was also forceful in suggesting the kinds of songs he was looking for. In one recording he tries to cajole the blues singer Blind Willie McTell into playing some of that "complaining music" about hard times, in spite of McTell's protests that he didn't know any.

By contrast, the English folksong collector Cecil Sharp was interested in isolating white Britishness. He travelled the country lanes of England seeking out rural workers for their unadulterated traditional material. In their songs he saw a distant reflection of the "merrie England" of myth. Sharp then travelled to America to document the survival of the English and Scottish tradition in the isolated communities of the Appalachian mountains. At the time, one out of every eight Appalachians was black, but Sharp dubbed black Americans "a lower race", recoiled from towns with too high a proportion of them, and concentrated only on those songs he considered pure British folk song.

Dave Harker and other writers have attacked Sharp for his bowdlerisations, and for sanitising working-class culture. Their criticisms take issue with his search for the voice of "common people", but they also suggest there is a true voice of the working class or proletariat, which Sharp had misrepresented. Sharp can certainly be described as a proto-fascist, but we should view him in context. He shared a widespread fascination with identifying the "roots of the nation", which he assumed to derive from a homogenous racial group. At its most extreme, this fascination led to the formation of deranged groups such as the British Israelites, who sought to prove that the British race was one of the lost tribes of Israel, and the Thule Society, precursors of the Nazis, who claimed that the origins of the German race lay in the mythical land of Ultima Thule, and who were also associated with the Völkisch movement.

Sharp's quest for the Aryan roots of folksong and his disdain for racial groups such as the Celtic Irish and Appalachian blacks was inherited from a European tradition that dated back at least to the Brothers Grimm, according to which, the rural "peasant", rather than the urban working class, was the repository of the remnants of a golden age. But rather than debating whether the urban or rural working class is more "authentic", the more relevant question is: is any tradition pure?

Must Sharp's and Lomax's racist views tarnish their wonderful song collections? Of course not. However, their views do tarnish the folk-song tradition, since they helped provide its modern foundation. By defining "folk" music as the most racially pure music, they established a heritage that was racist at its core.

By the late 1930s, these views were becoming unfashionable. For folk enthusiasts such as Alan Lomax, Woody Guthrie, Ewan MacColl and Pete Seeger a more functional definition was needed. Following communist theory, they defined folk music not by the song, but by its performance method. If it were sung without expectation of remuneration - while working, around a campfire, on a ship, among children - it was folk.

Such attitudes had been a minority view in the earlier folk movement. The song collector Dorothy Scarborough had given folk a far wider definition than her contemporaries Sharp and Lomax when she said: "A song that starts out as sheet music may be so altered ... by singers who learn and transmit it orally, as to become a folksong." Scarborough was happy to study material from both black and white musicians.

But look at what qualifies as folk under this more functional definition, and you are left with nothing but the detritus of pop songs. And that has always been the case. With a few exceptions (sea shanties, camp-meeting songs, playground songs, field hollers) the songs people sing while working or among friends were once performed by paid entertainers. Nick Tosches put it best: "Street balladry, the roots of traditional American music, was pop. The purest mountain airs, lustily pursued by sweaty, obsessive folklorists and concerned young things, were once the pop junk of urban Britain."

Have songs like R Kelly's I Believe I Can Fly and Green Day's Good Riddance (Time of Your Life) now become folk songs? Already they are sung in social gatherings without expectation of remuneration.

If separating folk songs from pop junk isn't based on racist or tribal ideology, and if a functional approach is incoherent, is there any way left to defend classifying some songs as one, and others as the other? Is there any such thing as a real folk song any more? Probably not. Let's jettison these old ideas of folk music, then. If we do, perhaps we can celebrate the inherent democracy of "pop junk", and more quickly overcome the racism in which our "folk" heritage has been steeped.

· Hugh Barker and Yuval Taylor are the authors of Faking It: the Quest for Authenticity in Popular Music (Faber, £14.99)